Photographic exposure under conditions to improve sensitivity of photographic emulsions

ABSTRACT

Under high vacuum, a photographic silver halide layer is evacuated of oxygen and moisture. For photographic exposures made under these conditions low-intensity reciprocity failure and dye desensitization of the silver halide are found to be substantially reduced or eliminated. This permits higher speeds with low-intensity exposures and permits greater spectral sensitization with dyes that ordinarily have a desensitization effect.

United States Patent William C. Lewis;

Thomas J. James, both of Rochester, N.Y. 736,650

June 13, 1968 Nov. 23, 197 l Eastman Kodak Company Rochester, N.Y.

inventors Appl. No. Filed Patented Assignee PHOTOGRAPI'IIC EXPOSURE UNDER CONDITIONS TO IMPROVE SENSITIVITY OF References Cited OTHER REFERENCES Vanselow et al., PSA Journal, Sect, B, March I952, pp. l i- 14.

Primary Examiner-Norman G. Torchin Assistant Examiner-Edward C. Kimlin Attorneys-W. H. .l. Kline, Bernard D. Wiese and Gordon L.

Hart

ABSTRACT: Under high vacuum, a photographic silver halide layer is evacuated of oxygen and moisture. For photographic exposures made under these conditions low-intensity reciprocity failure and dye desensitization of the silver halide are found to be substantially reduced or eliminated. This permits higher speeds with low-intensity exposures and permits greater spectral sensitization with dyes that ordinarily have a desensitization effect.

PHOTOGRAPIIIC EXPOSURE UNDER CONDITIONS TO IMPROVE SENSITIVITY OF PHOTOGRAPHIC EMULSIONS This invention relates to photography. ln one respect this invention relates to photographic exposure of silver halide photographic layers of substantial absence of oxygen and moisture, as in high vacuum.

PRIOR ART In 1934, Blau and Wambacher showed that the desensitization action of typical photographic silver halide desensitizing dyes such as Pinakryptol yellow is decreased by removal of oxygen and water, and other workers have confirmed their findings. Zum Mechanismus der Densensibilisierung photog raphischer Platten" by Blau and Wambacher-Wien in the Zeitschrzjft fuer wissenschaftliche Photographic, Bd. 33. Heft 8 (1934), pages 191 et seq.

Blau et al. exposed photographic plates coated with silver halide emulsion containing no dyes, and they exposed other plates coated with emulsions containing dyes that under ordinary exposure conditions have a predominant desensitization effect. When undyed plates were exposed after evacuation of oxygen and water in high vacuum l"=" torr), Blau et al. observed no significant change in sensitivity; when the dyed plates were exposed after evacuation in high vacuum, the desensitization effect of the dyes was practically eliminated.

One object of the present invention is to provide improved methods of photography comprising exposure of a photographic silver halide layer under conditions that are found to eliminate or significantly reduce low-intensity reciprocity failure. Another object of the invention is to provide a method for exposure of a photographic silver halide layer under conditions to eliminate the usual desensitization effect of dyes which also have a substantial spectral sensitization effect, thereby increasing the net spectral sensitization of the silver halide by the dye.

The foregoing and other objects of the invention will be better understood by reference to the following description of the invention which includes specific examples of preferred embodiments.

We have discovered that when a photographic silver halide layer has been degassed, as by evacuation in high vacuum so that substantially all oxygen and moisture are removed from the silver halide, the silver halide exhibits a marked decrease of, in some cases practically complete elimination of, low-intensity reciprocity failure upon exposure in the low-intensity range. This amounts to a speed increase for exposures by illumination in a range of low intensity at which significant lowintensity reciprocity failure usually occurs. This is found to be true for a variety of photographic silver halides as described herein.

in accordance with one aspect of the invention, a silver halide emulsion layer substantially free of oxygen and moisture is exposed to illumination which is in the low range of intensity that in ordinary atmospheric conditions causes significant lowintensity reciprocity failure; a significant speed increase is observed, due to reduction or elimination of reciprocity failure. Due to this speed increase, for a given intensity of exposure in this low range, the time required for optimum photographic exposure of the silver halide layer that is free from oxygen and moisture is substantially less than the exposure time required for the same silver halide layer in ordinary atmospheric conditions.

A convenient means for degassing the silver halide to remove oxygen and moisture is to place the photographic element in high vacuum for a time sufi'lcient to permit the oxygen and water to escape from the silver halide layer before photographic exposure. In the examples below, the vacuum is at 10"" torr. lt should also be noted that reduced pressure facilitates adequate degassing, which is demonstrated by first degassing the silver halide layer, then exposing in a nitrogen atmosphere at atmospheric pressure. It is convenient to photoexpose the silver halide while it is still in vacuum after degassing and that is our preferred mode, although the invention includes exposure of the emulsion at any pressure so long as the exposure is in the substantial absence of oxygen and moisture.

The speed increase obtained with low-intensity exposures under the recited conditions is useful in a number of different specific applications, as for example in astronomical photography where exposure times can be significantly reduced by exposing the camera plates in substantial absence of oxygen and moisture. Another specific application of the invention involves use of a silver halide emulsion that is substantially insensitive to ambient light conditions because of extreme reciprocity failure in the ambient light intensity range. A film, paper or plate coated with a layer of this silver halide emulsion can be handled openly in ambient conditions, e.g., in room light and ambient atmosphere, until the user is ready for exposure. When ready for exposure, the user degasses the silver halide layer and then it can be exposed photographically by the same ambient light. Thus, by elimination of low-intensity reciprocity failure the silver halide has been made sensitive to the same ambient light.

Following are specific examples describing in detail certain preferred embodiments and including several preferred modes for carrying out the invention.

EXAMPLE 1 A silver bromide emulsion is prepared as described by Spencer and Atwell, J. Opt. Soc. Am. 54, 498 (i964). The emulsion, which is not chemically sensitized and contains no addition of dye, is coated at a spread of 60 mg. silver and 125 mg. gelatin per square foot on a conventional polyalkylcne terephthalate film support. The coating is exposed through s step wedge in a sensitometer which can be evacuated when desired. The exposure is made to a 1,000-watt tungsten lamp filtered by a combination of one Wratten 36 and one Wratten 38A filter (which limits the exposure to the region of inherent sensitivity of the silver bromide) and operated at 3,000 K. at a distance of 150 cm. from the film. The exposed film is then developed for 5 min. at 20 C. in a solution of N-methyl-paminophenol sulfate 2.5 g., ascorbic acid l0.0 g., sodium carbonate monohydrate 500 g., potassium bromide 1.0 g., and water to make 1 liter. It is then fixed for 2.5 min. in Kodak F5 fixer solution, then washed and dried. Characteristic curves for several exposure conditions are shown in H6. 1. When the film is exposed on the sensitometer in room air at atmospheric pressure, the characteristic curve marked room air, 30- second exposure is obtained for an exposure time of 30 seconds. After the chamber containing the film has been evacuated for 16 hours to a pressure of 10" torr before exposure, the same exposure gives the curve marked vacuum, 30-second exposure. When the exposure time is extended to 2 hours, with the light intensity correspondingly reduced by a neutral density" filter, the correspondingly marked solid curves are obtained. It is clear that, for exposure of the film in air, speed and gamma are substantially less for the 2 hours exposure than for the 30 seconds exposure. The film in air exhibits conventional low-intensity reciprocity failure. In vacuum, however, speed and gamma are substantially greater for the 2 hours exposure than for the 30 seconds exposure, and the low-intensity reciprocity failure is no longer in evidence.

Example 2 A silver iodobromide emulsion is prepared substantially as described in French Pat. No. 1,497,202 without chemical sensitization or dye additions. This is coated on a polyalkylene terephthalate film base at a coverage of about 60 mg. silver and mg. gelatin per square foot. The coating is exposed as described in the preceding example with the results shown in Fig. 2. This emulsion shown substantially greater speed and gamma for exposure in air for 30 seconds than for exposure in air for 2 hours, yet in vacuum the emulsion shows equal or somewhat greater speed for the 2 hours exposure. The low-intensity reciprocity failure encountered with exposures in room air is not in evidence with exposures in vacuum.

EXAMPLE 3 EXAM PLE 4 A fine grain silver iodide emulsion is coated on cellulose triacetate film base and samples of the film are exposed for 30 seconds to a 1,000-watt tungsten lamp (3,000 K) through a step tablet having incremental steps of 0.2 density unit. The film is exposed at a distance 150 cm. from the lamp. The exposed film is developed for 27 minutes at C. in a solution of the composition: sodium bisulfite 5.0 g., pyrogallol 10.0 g., sodium sulfite 25.0 g., sodium carbonate monohydrate 50.0 g. potassium bromide 0.1 g., and water to make 1 liter. No image is obtained when the film is exposed in air. When the film is exposed the same after 16 hours evacuation to 10 torr, eight steps are visible on the developed strip.

in still other embodiments of the invention, a silver halide layer that has been evacuated of substantially all oxygen and moisture, and contains a dye which has significant spectral sensitization effect on the emulsion, is photoexposed in substantial absence of oxygen and moisture. Under these conditions the usual desensitization effect, which has generally been considered an undesirable concomitant effect of nearly all sensitizing dyes, is substantially reduced and in some cases practically eliminated. In the conventional practice of dye sensitization, a selected dye is added in an amount found to give optimum sensitization; by that is meant an amount found to give the maximum spectral sensitization without intolerable desensitization and fog. See Mees and James, The Theory of the Photographic Process, Third Edition (1966), pp. 198-273 and particularly pp. 257-261. However, when a photographic emulsion is to be photoexposed under the conditions of the present invention, the selectedsensitizing dye may be used in greater proportion in the emulsion, because the usually expected desensitization effect has been substantially reduced. In some embodiments a spectral sensitizing dye may be used in concentrations high enough to give monomolecular layer coverage of the entire silver halide crystals without an intolerable amount of desensitization. Reduction of the desensitization effect according to the invention gives a substantial increase in the available optimum sensitization of an emulsion with almost any spectral sensitizing dye. We have even found, by drastically reducing the desensitization effect of some dyes that are usually considered as being predominantly desensitizers, there is revealed a useful spectral sensitization efiect in such dyes, and this latter effect becomes the predominant effect of the dye in certain embodiments of the invention.

EXAMPLE 5 A coating of the emulsion described in example 2, without chemical sensitization, is prepared with the addition of 985 mg. per mole silver halide of the sensitizing dye, 5,5'-dichloro- 3,3,9-triethylthiacarbocyanine bromide, sufficient to give approximately monolayer coverage of the silverhalide grains. Upon exposure in air for seconds and development under the conditions described in example 1, a relative speed (measured at a density of 0.1 and gamma of 0.6 and 0.12, respectively, are obtained. Upon exposure after evacuation of 16 hours at 10" torr, the speed and gamma are 36.5 and 0.93 respectively.

EXAMPLE6 The emulsion coating described in example 5 is exposed through a Wratten 16 filter instead of the Wratten 36+3 8A combination, so that the exposure now is confined to the spectrally sensitized region. The relative speed and gamma for exposure in air are 2.25 and 0.20, respectively; and for exposure after 16 hours of evacuation at 10''" torr are 280 and 0.70, respectively.

EXAMPLE 7 A coating of the emulsion described in example 2 without chemical sensitization is prepared with the addition of 200 mg.

per mole silver halide of the dye:

posure after 16 hours of evacuation, a relative speed of 178 is obtained.

p-toluono sultonato EXAMPLE 8 A silver bromide emulsion is prepared as described by Spencer and Atwell, J. Opt. Soc. Am. 54, 498 (1964). The

emulsion is not chemically sensitized. Before coating, 200 mg.

of the dye described in example 7 was added per mole silver bromide, and the emulsion was coated at a spread of mg. silver and mg. gelatin per square foot. The coating, exposed in air for 30 seconds as described in example 1, gave no image. When exposed after 16 hours of evacuation, a relative speed (measured at density 0.1) and gamma of 89 and 0.8, respectively, were obtained. When exposed in air as described in example 6, no image was obtained. When exposed afier 16 hours of evacuation, a relative speed and gamma of and 0.85, respectively, were obtained.

When the emulsion was given a 0.5-millisecond exposure to a Strobonar flash lamp instead of the 30-second exposure, evacuation produced no increase in speed or gamma.

EXAMPLE 9 The silver bromide emulsion of example 8 is chemically sensitized by digestion for 1 hour at 65 C. in the presence of 20 mg. thioureadioxide per mole silver. The dye of example 7 is added as in example 8 and the emulsion is coated as in that example. When exposed in air for 30 seconds as described in example l, a relative speed and gamma of 1.45 and 0.16, respectively, are obtained. When exposed after 16 hours of evacuation, a relative speed and gamma of 178 and 1.6 are obtained. When exposed in air as in example 6, a speed and gamma of 1.6 and 0. 1, respectively, are obtained. When exposed after 16 hours of evacuation, a relative speed and gamma of 295 and 1.6 are obtained.

EXAMPLE 10 The emulsion described in example 1 is sulfur-sensitized and 200 mg. of the dye s f 0,N-

+ l ethylsulfate Et are added per mole of silver halide prior to coating. Upon exposure for 30 seconds in air and development as described in example 1, a relative speed and gamma of 0.5 and less than 0.1 are obtained. Upon exposure after 16 hours of evacuation, a relative speed and gamma of 100 and 1.15, respectively, are obtained. Upon exposure in air as described in example 6, only a trace image is obtained. Upon exposure after 16 hours of evacuation, a relative speed of 17.8 and gamma of 1.05 are obtained.

EXAMPLE 1 l A coating of the emulsion described in example 2, without chemical sensitization, is prepared with the addition of 800 mg. of the merocyanine dye EXAMPLE 12 A coating of the emulsion described in example 2, without chemical sensitization, is prepared with the addition of mg.

of the dye N N N per mole silver halide. Upon exposure in air for 30 seconds and development as described in example 1, only a trace image is obtained. Upon exposure after 16 hours of evacuation, a relative speed of 126 and gamma of 1.7 are obtained. Upon exposure in air as described in example 6, no image is obtained. Upon exposure after 16 hours of evacuation, a relative speed of 16 and gamma of 1.25 are obtained.

EXAMPLE 13 A coating of the emulsion of example 2 is prepared with the addition of 985 mg. 5,5'-dichloro-3,3,9-triethylthiacarbocyanine bromide. Upon exposure for 30 seconds through a Wntten No. 16 filter and a 0.8 neutral density filter to a 1,000-watt tungsten lamp (3,000 K.) at a distance of cm. from the film, a speed of 2.0 (arbitrary units) is obtained when the exposure is made in room air and a speed of 315 when the exposure is made at 10" torr. When the film which has been held in vacuum for 16 hours at 10" torr is returned to room air before exposure, the speed has decreased to 31.5 after 5 minutes in air, and 12.6 after 1 hour in air. FIG. 4 in the drawings is a group of curves showing the original speed and gamma of the coating (curve marked room air), the speed and gamma for exposure in vacuum after evacuation for 16 hours at 10" torr vacuum, and the speed and gamma for exposures made 5 minutes, 1 hour, 4% hours, 26% hours and 96 hours, respectively, after the evacuated film has been restored to room air.

EXAMPLE 14 All of the foregoing examples are repeated except, instead of the photographic exposures in vacuum, the films are first evacuated as described and then the pressure is raised by admitting only pure dry nitrogen into the vacuum chamber until atmospheric pressure is reached. Time is allowed for equilibration, then the films are exposed as described. In every instance the results of exposure in nitrogen are practically the same as the results of exposure in high vacuum.

The vacuum exposure apparatus used in the foregoing examples has a vacuum chamber formed by an aluminum bell jar, the rim of which rests on a flat, polished steel pump plate. A gasket seals the rim of the jar against the fiat plate. The pump plate is equipped with a valved port for evacuation of the chamber and another valved port for admission of gases to the evacuated chamber. The bell jar is equipped with a sealed clear glass window which transmits light from an external source into the chamber. A frame stands on the polished plate, inside the chamber, and this frame is equipped with means for holding a photographic film at a position facing the window of the bell jar. Light transmitted through the window into the bell jar impinges directly upon the film surface. The window is equipped inside the chamber with means for holding a flat step-wedge plate in the path of light entering through the window. The window is equipped on the outside with a sliding shutter for opening and closing the window to light. Any suitable light source may be employed outside the vacuum chamber for projecting light through the window, into the chamber and onto the film. Any suitable high-vacuum pump may be used to evacuate the chamber through a valved port in the pump plate. We have found it is advantageous to provide several film-holding planes on the frame and to provide externally operated means for rotating the frame inside the chamber to several positions, so that each of the film-holding planes on the frame can be rotated in successive turns into and then away from the path of light entering through the window. This permits a single separate individual exposure of each of the several films on the frame without need to open the chamber (and hence lose vacuum) after each photographic exposure. For admitting gas into the chamber, the second valved port in the pump plate is opened, either to atmosphere to admit air or to another gas source to admit a selected gas.

Silver halide layers that are useful in various aspects of the invention may be silver halide emulsions or other kinds of photosensitive silver halide coatings. We especially prefer to use thin layers because they are easier to degas, but any silver halide layer capable of being degassed will be operable in the process of the invention. The silver halide may be any suitable single halide or combination of silver halides such as silver bromide, silver chloride, silver iodide and combinations such as iodobromide, chlorobromide, iodochloride, etc.

The invention may be practiced with silver halides of various grain structures and grain sizes. The silver halide layer may be an emulsion layer with gelatin or other suitable binder, e.g., polyvinyl alcohol and the like. The silver halide may be coated on any suitable paper, film or plate support and we prefer bases that can withstand high vacuum without damage by outgassing. Glass plates and polyethylene terephthalate films are especially suitable.

The invention may be practiced using photographic silver halide layers containing any of the usual addenda for modifying the photographic and physical properties of the layers. Such addenda might include chemical and dye sensitizers, antifoggants, gelatin plasticizers, coating aids and the like. The invention may be practiced with color-recording elements, although greater care is necessary to avoid damage by outgassing as the water and oxygen are being removed from the multiple layers of the element.

Although the invention has been described in detail with particular reference to preferred embodiments thereof, variations and modifications can be effected within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.

We claim:

I. A photographic method comprising (a) degassing an unexposed photographic element comprising a support and a spectrally sensitized photographic silver halide emulsion layer,

by evacuation at a pressure of l0 torr or less until said emulsion layer is substantially free of oxygen and moisture, and (b) exposing, at a pressure of 10" torr or less. the photographic element to low-intensity illumination.

2. A photographic method as in claim 1 wherein said photographic silver halide emulsion is chemically sensitized before said degassing.

3. A photographic process for exposing a silver halide layer, spectrally sensitized with a dye, to low-intensity illumination whereby low-intensity reciprocity failure and desensitization are substantially eliminated, the process comprising the steps of l) removing, by high vacuum, substantially all free oxygen and moisture from an unexposed silver halide layer, and (2) thereafter exposing said silver halide layer to lowintensity illumination in the substantial absence of oxygen and moisture.

4. An improved method of photography defined by claim 3 wherein said exposing step is carried out in high vacuum,

5. A method defined by claim 3 wherein said exposing step is carried out in an atmosphere of nitrogen.

2;;3? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,622,32LL Dated N v m 23, 97

Inventor(s) William G. Lewis and Thomas J. James It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 7, "of Substantial should read in Substantial"; line 25, "1O '7E'6 torr'" should read "10 torr"; line 72, "10 '7E'5 torr" should read "10" torr'" In each of line 52 of Column 2, lines 29 and 7h of Column 3, lines 8 and 31 of Column Lt,

lines LL, 5 and 11 of Column 6, lines 1 and 3 of Column 8, "lO torr" should read "10 tor-r".

Si 'ned ne. sealed this L th day of July 1972.

(SEAL Attest:

igiihljlf' l'li M.FL1TC1I1ZR, JR B- 1T GOTTSCHALK Attcrxtlng Offlc er Commissioner of Patents 

2. A photographic method as in claim 1 wherein said photographic silver halide emulsion is chemically sensitized before said degassing.
 3. A photographic process for exposing a silver halide layer, spectrally sensitized with a dye, to low-intensity illumination whereby low-intensity reciprocity failure and desensitization are substantially eliminated, the process comprising the steps of (1) removing, by high vacuum, substantially all free oxygen and moisture from an unexposed silver halide layer, and (2) thereafter exposing said silver halide layer to low-intensity illumination in the substantial absence of oxygen and moisture.
 4. An improved method of photography defined by claim 3 wherein said exposing step is carried out in high vacuum.
 5. A method defined by claim 3 wherein said exposing step is carried out in an atmosphere of nitrogen. 